Telegraph system



Aug. 28, 19,51 P. LE cALvEz TELEGRAPH SYSTEM 2 Sheets-Sheet 1 Filed Aug. 25, 1947 523mm m m.-

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Aug. 28, 1951 F. LE ALvEz -TELEGRAPH SYSTEM i Sheets-Sheel 2 Filed Aug. 25, 1947 mmm! com .cui 9550A NNN Baca RON INVENTOR Philippe Yves Marie Le Galvez 5232". :012m @533mm Patented Aug. 28, 1951 UNITED STATES PATENT OFFICE TELEGRAPH SYSTEM Application August`25, 1947, Serial No. 770,416

\ In France April 28, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires April 28, 1963 i9 Claims.

This invention relates to wireless telegraph systems, and more particularly to duplex radio and like telegraph systems of the printing telegraph or perforated band receiver type and has for` its object to provide improved telegraph sys1 tems with a very high degree of security, i. e., a very high degree of immunity from errors due to interference or like causes.

The invention preferably uses the known system of harmonic telegraphy in combination with a" code which transforms the character (letter, figure or other element) to be transmitted into a group of (n) elemental signals, each such elevmental signal -being characterized by the transmission of a predetermined one of a .pair of frequencies allotted to each signal, all the frequencies of the various pairs preferably, though not necessarily modulating, as in harmonic telegn raphy, a common carrier wave. In the example to be given'hereinafter, a 5-unit code (i. e., one in which 11:5) is used.

As will be seen later, the invention offers the important advantage of automatically obtaining adjustment of the traffic speed according to the actual conditions of propagation. This advantage is obtained by reason of a control of the transmitter by the translator in such manner that the transmitter can only transmit a fresh signal when the previous signal transmitted has been appropriately recorded by the receiver.

In a system in accordance with the invention, a group of modulations transmitting a character is preferably transmitted in a continuous or uninterrupted manner until the arrival of a control signal sent out by the receiving station by means of an associated transmitter effecting duplex operation cn another wavelength. On the arrival of this control signal, the transmitter immediately changes its modulation so as to transmit the next character. The control signal itself preferably comprises one or the other of two modulation frequencies, called control frequencies, of the wave radiated by the associated transmitter, each of said two control frequencies being used in turn.

Furthermore, the transmitting station receives an additional modulation, independently of those used for the character to be transmitted., by means of one or the other of two frequencies, called starting frequencies or identification frequencies, and serving to dierentiate each character successively transmitted. Like the others, this starting frequency is continuously transmitted and suddenly changes as soon as the transmitting station has received the control Cil signal which is manifested by the change of conn trol frequency affecting the receiver which is associated with it for duplex operation. It will therefore be seen that finally, owing to the duplex coupling, each transmitter is modulated lby the signalling, control and starting frequencies. All of these frequencies may be transmitted in any convenient appropriate manner, e. g., by. normal amplitude or single side band modulation, frequency modulation, phase modulation and so on.

I will now describe one channel of a duplex transmission system in accordance with the invention. It is to be understood that the other channel operates in a similar manner.

In this system;

(c) Each signal transmitted comprises a combination of n frequencies each belonging to a group of two frequencies, while the receiving apparatus comprises n groups of two relays arranged in such manner that each frequency transmitted causes the energiaation of one and only one relay in the` corresponding group.

(b) When one and only one relayis energized in each of the n groups, the signal transmitted is recorded and a control frequency is sent to the transmitter which has remained locked since the preceding transmission in order to cause the next signal to be transmitted.

A general system as above described would still have the drawback that general fading causing the same combination or the same control signal to disappear and then reappear, might either cause repetition of the same letter or signal in the receiver, or cause a fresh combination to be prematurely transmitted. In order to avoid this diiiiculty the invention provides that:

(l) Each combination of n frequencies is accompanied by another identification frequency, called the starting frequency, which is not the same for any given signal transmission and for the next transmission, two frequencies alternately fulfilling this function.

(2) The control frequency -is similarly changed after each recording of a combination the re ceiver, two frequencies being used alternately for this purpose.

(3) As for the reception of signalling fre quencies, the reception of the identification frequencies and of the control frequencies is effected by a group of two relays arranged in such a manner that each frequency transmitted produces the energization of one and only one relay, the armatures of said relays controlling a startu ing circuit, either for transmission (at the transmitter), or for recording (at the receiver), which circuit is only energized when one and only one of said relays is operative.

It will be seen that a system as above described will automatically adjust its own tran speed according to the prevailing conditions or propagation. On the other hand, the direction of the traflic depends entirely on the receiving position which can adjust the delay of sending back the control frequency, and even stop such sending back, and thereby stop the trans-- mission, Finally, the reliability with which each signal is received makes it possible, by means of rotary switches which move forward by one step after each signal. to multiply the communication in an almost unlimited manner.

The invention includes within its scope all embodiments of systems as above described, and in particular those showing one or more of the following features:

(a) The transmitters and theI translators are respectively connected in parallel with the variu ous levels of a rotary switch.

This expedient is well known per se its purpose being to use the full capacity of a wireless channel by transmitting in turn the characters of diierent messages.

(b) The rotary switches have a shaft which is connected by a sliding clutch to a continuous rotating driving shaft, but are retained by an escapemen-t wheel, the pawl or equivalent device of which is actuated by a relay in such manner that each oscillation of the pawl or the like feeds the rotary switch one step forwards.

(c) Successive energizations of the starting `circuit of the transmitting apparatus alternately change the polarity of the signal transmitted to a control line.

(d) The starting circuit is broken during the operation of the rotary switch.

(e) The starting circuit includes a polarised relay, the armature of which alternately transmits a predetermined polarity to two xed contacts respectively connected to the even and odd contacts f a ring of contacts on the rotary switch, while the brush or wiper arm of said ring of contacts is connected to a hold-on relay connected to a point of opposite polarity, said hold-on relay having a dead con-tact in the starting circuit.

At the transmitting end (f) The combinations to be transmitted are each recorded alternately by one or other of the relays of a group of two relays, the switching from one of the relays to the other taking place at each reversal of polarity of the control line. 1

(g) The relay of the group which is not in the recording position is placed in the transmitting position, the registering of -the next combination being thus effected during the transmis sion of the previous combination, while the switching of the transmitting circuit from one relay to the other is also effected at each reversal of polarity of the control line.

(h) The two windings of the two relays of the same recording group are arranged in parallel and connected to the xed contacts of a polarised relay which is connected to the control line and the armature of which is connected to a wiper arm of the rotary switch belonging to one of the contact rings to the contacts of which the transmitters are connected in parallel.

(i)` The armatures of the two relays of the same recording group are connected to the xed contacts of a polarised relay which is connected to the control line and the armature of which is 60.10,u

nected to a control circuit of the transmission of one or other of two signalling frequencies, whereas the xed contacts of said relays are respectively given two opposite polarities.

(j) The control circuit of the identification transmission isA connected to the control line.

(Ic) The pawl or the like escapement member of the rotary switch is actuated by a polarised relay connected to the control line.

(Z) The rotary switch is provided with a special ring of contacts for actuating the relays controlling the electromagnets for moving the transmitters to the next level, 'so that after having operated, said transmitters may prepare the next combination. A

At the receiving end- (m) The armatures of the two relays of each receiving group control a receiving circuit which is only energized when one only of said relays is operative, the direction of the current being different according as to whether one or the other of the two relays is operative.

(n) The receiving circuit of each group con tains the winding of a polarised relay, the armature of which is connected to one of the recording wiper arms of the rotary switch, and imparts to said arm one polarity or the other according to the direction of energization of said relay.

(-o) The recording circuit of each group imparts, when it is energized, a predetermined polarity to the Winding of a chain relay which is connected to the wiper arm of the hold-on relay and is provided with a hold-on circuit including a dead contact of an end relay.

(1D) The successive energizations of the starting circuit of the receiving apparatus alternately change the polarity imparted to a control line, the other end of which is grounded through the chain contacts, in such a manner that a current only passes through said line when all the chain contacts have been closed.

(q) The control line contains in parallel a polarised relay, the energization of which in one direction or the other determines the sendingr towards the transmitter of one or the other of the control frequencies according to the direction of its energization, and an unpolarised relay, the energization of which produces the recording of the combination received.

(r) The recording of the unpolarised relay produces the energization of an end relay which is provided with a hold-on circuit connected to that of the hold-on relay by an operative conn tact.

(s) The energization of the end relay completes the energization circuit oi a polarised relay connected to the control line, said polarised relay determining by its energization in one direction or the other that of the electromagnet controlling the rotary switch.

(t) The rotary switch has a special ring of contacts for controlling relays which start on the next level the energization of the reading electromagnet of the translating device.

(u) A switch arranged in the circuit for sending back the control frequency either prevents this sending back or operates step by step.

Accompanying drawings show, by way of example only, one embodiment of the invention. In the drawings:

Fig. 1 is a schematic diagram of the transmitting station.

Fig. 2 is a schematic diagram of the receiving sta-tion.

1n the present state of telegraph technique,

it is advantageous to use harmonic telegraphy systems both at the transmitting end and at the receiving end; copper oxide modulators are particularly advantageous in this case owing to their practically zero time constants as compared to the duration of operation of a relay.

The automatic transmitters and the translating devices to be used may beof any type. In this example, in which it is assumed that the 5-unlt Baudot code is employed, it is only necessary for them to be capable of transmitting or registering 5 code signals simultaneously. ln order to give a specic example, the frequencies may be numbered from 1 to 1G, the even nurnbered frequencies corresponding to the sign and the odd numbered frequencies to the sign. The transmitters are modulated at telephone frequency by these combinations f frequencies. y

The rotary switches are preferably, as shown, of the pawl-escapernent type driven by a friction clutch. The optimum number of levels is twelve, since this number enables the greatest number of combinations to be obtained and 1, 2, 3, 4, 6 or 12 channels to be obtained on the same communication means by simply connecting the transmitters and the translating devices in parallel.

The contruction of the apparatuses shown will be described at the same time as their operation, in order to avoid useless repetitions. It is to be noted that the armatures of the relays are shown throughout in their inoperative positions.

Transmission (Fig. 1

Referring to Fig. 1 of the drawing in detail there is illustrated a schematic diagram of connections of a transmitting station including a control signal receiver. This signal receiver employs an antenna |55 connected tothe receiver |55 comprising an ampliiier and detector for receiving and detecting the return control signal which is fed from the output of the apparatus |56 over the conductor |55 to the two filters |51 and |58. The outputs of the filters |51 and |58 are connected to the relays I and 2, respectively.

It can thus be seen that, depending upon whether the received control signal is constituted by one or the other of the modulation frequencies, only the corresponding iilter |51 or !58 will feed a signal to the corresponding relay or 2.

It will be assumed that transmission has already begun and that a combination has already been transmitted. As explained, the next combination can only be transmitted when the control frequency has been received at the transinitting station. At this instant, one and only one of the two polarised relays I or 2 controlling the relay 3 is excited. It will be assumed for example that it is the relay I.

The armature of the relay I moves into its operative position, which has the effect of energizing the telegraph relay 3, which is adjusted neutrally, through the following circuit: the plus terminal of the battery |55), armature la (operative), dead contact 4a of the relay fl (which will be described hereinafter), armature 4b of said relay 4 (inoperative) windings of the relay 3, armature 2d of the relay 2 `(inoperative) to the minus terminal of the battery |59. Owing to its excitation, the relay 3 moves its armature 3a from one position to the other, and connects the control line 5 connected to the armature 3a to the negative terminal assuming that it was pre'- viously connected to the positive terminal or conversely.

lit will be understood that at the next control signal transmission received by the receiver |56, the relay 2 will be energized through the lter |58, which will have the effect, similarly of energizing the relay 3 but with a currentof opposite direction, which'will have the effect of connecting the control line 5 to the plus terminal of the battery through the armature 3a, since it was previously connected to the negative terminal.

The system of the two relays I and 2 is thus characterized by the fact that the two armatures Ia and 2a, which are respectively connected to the windings of the relay 3, are in contact when inoperative with stationary contacts which are connected to the same pole and, when operative they `are in contact with stationary contacts of opposite polarity; the whole arrangement being such that if one only of the two relays I or 2 is energleed, the relay 3 is energized in a predetermined direction which depends solely on which of the two relays I or Z is energized, whereas if said two relays I and arev both inoperative or operative, the relay 3 is not energized. t will be seen, furthermore, that the armature 3a will not change its position if the same relay I or 2 were to be energized twice in succession. It will therefore be seen that this armature sa will not change its position either as a result of static, or of fading.

The change of polarity thus effected on the control line 5 by the reception of a control frlquency has various effects:

A first effect is to produce energization of the electromagnet 6 for releasing a rotary switch driven by a continuously rotating drive shaft through a clutch, but normally stopped by the paw] sa associated with the escapeinent wheel 5b. The movement of the pawl from one position to the other releases a tooth of the wheel db, in the .well-known manner, and consequently feeds the rotary switch one step forwards.

The rotary switch has as many banks (designed by dd) as the code used for the transmission of the characters has units, i. e. elemental signals identifying the character and, in addition, two

.banks Ul and lic, the functions of which will be explained hereinafter. The wiper arms of all these banks are on the same shaft which-is controlled by e and therefore move together'.

The banks 8d have the same even number of contacts and there are as many translating devices Tn as there are contacts, i. e. messages transmitted, or which can Ice transmitted, simultaneously. Each translating device Tn is con nected to all the contacts of the same order of the banks ad and its function is to give to each of the connection wires a or polarity. As stated, the whole of these pola-rised signals char acterise the signal to be transmitted.

In short distance communication, it might happen that the next control frequency is received before the end of the rotation through one 5, step thus produced. It is to avoid this drawback i that the hold-on relay Il has been provided. The

winding of this relay is connected to one oi the wiper arms flo of the rotary switch. The contacts of the ring lid corresponding to said arm are alternately (in even and odd numbers) connected to the nxed contacts 1a and 'ib of a polarised relay 1 connected in parallel with the relay 3. The armature l'c of said relay 'i is grounded.

When the control frequency is received, the relay 1 is energized and its armature 1c: changes position. It thus grounds the even numbered contacts, for example, of the ring id assuming that previously ground was connected to the odd numbered contacts (or conversely). The wiper arm 4c being at this instant on an even numbered contact, the relay i is energized and breaks at llc-4b the circuit of the relays 3 and l. In this manner, no fresh energization is possible for these relays until the end of the rotation. When this occurs, the wiper arm to touches the odd numbered contact which is not grounded, so that the relay 4 is released and the reception of the next control frequency becomes possible.

The apparatus described, which operates in this example according to the 5-unit Baudot code, comprises ve groups of two relays 8 and lII which are connected on the one hand to the control line 5 and on the other hand to ground (only two of these groups are shown). The relays 8 and I are therefore energized in one direction or in the other, according to the polarity to which the line is connected.

In each group, the armature 5a is connected to a wiper arm of the rotary switch. There are therefore live such wiper arms and five corresponding rings oi contacts. The transmitters Tn are connected in parallel with the corresponding contacts of the different rings; for example, for the contacts of row n, the transmitter shown at Tn transmits (as will be described) the previously prepared combination of frequencies. The stationary contacts 3b and 8c cooperating with the armature sa are respectively connected to the windings of two relays 9 and ||J connected in parallel, the other ends of said windings being grounded. The armatures 9a. and Ilia of these relays s and iii are respectively connected to the fixed contacts im and IIIJ of the armature Hc of the relay Ii, and, the whole arrangement of relays s and II is such that, when the relay s is fed by the armature 8a, the armature Ia is connected to the armature IIe, while when on the contrary, the relay I is fed through the armature sa, the armature 9a is connected to the armature I ic.

Thus while one combination is being recorded on the relays e the combination which had previously been recorded on the relays lil is transmitted.

Finally, the reversal of the polarity of the control line a produces at the same time the reversal of the control oi the identification or starting frequency by the relays I2@ and I 2b, the position of which is controlled by this polarity, their common point being connected to the line 5. These identification or starting frequencies are supplied by the generators |3| and E32 that are selected by the energization of the relays I 2a and |2b, respectively.

Thus, at the instant when the control frequency is received at the transmitter by the receiver l, this reception produces a rotation by one step of the rotary switch controlled by the pawl 6a and escapement Eb, the recording of the next combination to be transmitted, the transmission of the combination previously recorded, and the simultaneous transmission of the starting frequency. It is apparent that according to the polarity transmitted by the armature IIc, the relays I3 produce the transmission of one or the other of the signalling frequencies of the pair corresponding to the group considered. Each group of relays I3 is connected to select frequencies generated by a pair of generators such as I2I and |22, for example.

A special ring of contacts of the rotary switch, cooperating with the wiper I4, serves for the energization of the relays Rn which ground the electromagnets for transferring the transmitters to the level (n+1), so that immediately after having operated, they may prepare the next combination. The rotary switch therefore has in all, in this example, seven levels of contacts. In the drawing, only two groups of relays 8 to II are shown, the three others being similar thereto.

It will therefore be seen that the relays Rn are connected to the contacts oi wiper I4 with a shifting of one contact in the direction of the movement of the rotary switch. As soon as the relay R11. has released its armature, the translating device T11. prepares the next combination.

1n the case of difficult communication, in order not to load the transmitter, which consists of the carrier generator I5 l modulator |52 and amplifier |53, by supplying it with six frequencies simultaneously, plus a seventh for the control of the other channel, it is advantageous to interpose between the armatures I Ic and the frequency control relays I3a and I3b a distributor rotat= ing at an arbitrary speed which controls the transmission of the frequencies in` any order. The speed of rotation of this distributor is obviously only limited by the width of the bandpass filters and the interval between the frequencies.

The relays |261. and |2b control the transmission of the starting frequency generated by the generators I3I and |32, the relays |20, and 31) those of the signalling frequencies supplied by generators such as |2I and |22. When the polarity of line 5 is such as to cause the relay 12a to cause its armature I2C to connect the generator |3| to the line |88 the identication or starting frequency supplied by the generator ISI is fed to the modulator |52 of the transmitter. On the other hand if the opposite polarity is supplied by the line 5 the relay |2l2 causes its armature |2d to connect the generator 32 to the line |08 so that the dentincation or starting frequency supplied by the generator |32 is connected to the line |08 to the modulator E52. The relays I3a and |3b function in a similar manner to connect the generators |2| and |22 through the line |06 to the modulator |52. These generators |2I and |22 generate frequencies which comprise the elementary signals and in the present example there are ve groups of such generators, each consisting of two generators 22| and |22. Thus there are also i'lve groups of relays |3a and |3b, one for each group of generators.

In accordance with usual telegraphic practice, the supply battery acting as a common source for the various circuits shown has its mid-point grounded. This is the case both in Figs. l and 2. The various leads connected in the drawing to positive or negative or to ground are therefore connected to the corresponding poles of the battery or to its mid-point.

Receiving emi (Fig. 2)

The action at the receiving end will now be described with reference to Fig. 2.

The receiving station consists of an amplifier and detector receiving apparatus 2|5 connected to the antenna 2|4. The output of this apparatus is connected by way of the conductors 2c?, and 2 to the lter sets 204 and 2B?, respectively. Five groups of filter sets such as 264 are em- --relays is energized.

ployed `for selecting the frequencies used for transmitting the elemental signals. 'The iilter set 201 which is connected to the apparatus 2|5 bythe conductor 2|! is employed for selecting the identification or starting frequency. Each of the filter sets '204 consist of a pair of filters 20| and 202 for selecting two different frequencies and the outputs of these filters are connected to energize the relays ii! and 20, respectively. The armatures of the relays |9a and 20a are connected to the conductors 209 and 2|0, respectively. A pair of contacts is associated with each oi the armatures itc and 20a and one contact of each of these pairs is connected to the positive terminal of the battery 203 and the other contact of each of these pairs is connected to the positive terminal of the battery 203. A central tap on this battery 203 is grounded. The filter set 207 is similar to the filter set 20d. In the case of the lter se; 207 there is provided a pair of filters 20M and 20th tuned toselect the identification `or starting frequencies. The outputs of the lters 2010i and 20719 are connected to energize the relays I and IS, respectively. These relays l ii and IS are provided with armatures Ida and |Sa and each of these armatures has associated herewith apair of contacts, one of these contacts oi each pair being connected tothe negative terminal of the battery 201e and the other contacts of these pairs Vbeing ,connected to the positive terminal of this battery. A mid-tap of this battery is grounded. The armatures ld and |50. are connected to the lines 2|,2 and 2|3, respectively. The line 2|2 is connected to one side of the relays I1 and I0 and the line 2| 3 is connected to the Vcontact 32a of the relay 32 to the other side of the relays il and i5. It will therefore be seen that the receiving apparatus |25 after detection feeds a set of frequencies to a harmonic telegraph-board si for controlling various relays.

The starting frequency is received by one `of vthe two relays shown at i5 and I6. The relays are arranged in a manner similar to that described with reference to the relays and 2 which, in the transmitter, receive the lcontrol `frequency, i. e. the armatures |5a and lita are connected to the winding of a relay --and to that ci a relay Iii, while the xed contacts |5b, |019 and |50, Hic, are respectively connected to the two poles of the battery.v In the normal position, no current therefore ows fthrough the windings of the relays i0 and l1, because the armatures Ia. and Ita are on contacts of the same polarity. Also no current flows if the two `armatures become operative at the-same time.

Vlt will, however, be obvious that a current of predetermined direction flows through said windings when'only one of the armatures |5a `Iia becomes operative. This arrangement is identical to the previous one and is for the same purpose.

The signalling frequencies are each received by one of the relays of a group ci two, it and 20, in a similar manner to that describedfor the reception of the starting frequency. yCcnsequently, in normal conditions of propagation, in each group of relays i9 and 20 only one of said It follows that the tele1 graph relay 2| indicates'by the position of its VarrnatureZld the direction of the `current which is flowing through its windings, but `remains insensitive to fading and to static. The combination is therefore recorded by the five relays such With each relay 2| are associated three relays 22, 23 and 24. The relays22 and 23 are respecf tively connected to ground and to the armatures liia and 20a and are so arranged as to supply a predetermined polarity to the winding of the relay 2li when one of said two relays 22 or 23 is operative. Consequently, when one of said relays 22 or 23 is energized, the relay has grounded the even or odd contacts of the bank iid which contact at that moment with the corresponding wiper arm (itself connected to the windings of the relays 24), the relay I2li is energized through the following circuit: battery, armature 23a (operative) armature 22a (inoperative), winding of the relay 24, dead contact 25a of the relay 25 (which will be described hereinafter), wiper arm ila, armature |1b oi il'v 'to ground. Use has been made, in this example, of two polarised relays S22- 23 instead of a single non-polarised relay, for the purpose of speed'of operation.

As the relay Zil is energized it closes a hold-on circuit for itself through the operative contact 24a and its second winding. `It also closes a chain contact 24h and at 2de it opens the circuit of therelay 2| in order to prevent, as disclosed for contact 4b in the transmitter, the arrival of a fresh combination from disturbing the recording oi the previous combination.

There are as many groups of relays lil- 2d as there are units in the code employed. In the drawing only one group is shown. lAs shown in the figure, the contacts 26h of the 1i relays 24 are connected in series.

When the five chain contacts such as 2th are closed, the ground 24o is connected to the relays 25 and 2'! in parallel. The windings of these two relays are connected to the armature |81; of the relay i3 which armature moves as soon as the starting `frequency arrives. Consequently, the relays 20 and 2T are energized and the armature .2 6a of the relay 26 changes position. The fixed contacts of this armature are connected to the `opposite poles of a supply battery of the circuit 23 controlling the transmission Aofthe control frequency. This frequency is therefore i1nmediately changed, thereby enabling, as previously explained, a fresh combinationto `be transmitted. The polarity supplied to the circuit 28 `actuates one or the other of the relays 5| or Eilawhich control the modulation of the transmitter lby means of one or the other Aof the `control frequencies supplied by the generators `222 and 223 so that the carrier wave radiated by the transmitter transmitting in the opposite channel carries one or the other of these control irequencies. The transmitter consists of the modulator 225 connected to the carrier frequency generator 220 and to the amplifier '221 which in turn is connected to Athe antenna 220. The modulator 225 is also connected by the line 22d to the armatures of the relays 5| and 5i a..

At the same time as one Aor the other of the relays `5| or Bla is energized through the circuit 28, the unpolarised relay 2'! moves into itsioperative position and, through its operative contact 27a and the dead contact y25h of the relay 25 (which will be described hereinafter grounds the electromagnets of the translating l devices such as Tn. It willbe obvious that only the translating device Tn, on the evelof which the rotary switch is stopped, `records the combination of the five relays `2 i. Y

21a (operative) the end relay 25 is excited through the following circuit: ground, operative contact Zla, inoperative contact 25D, winding of the relay 25 to battery. When energized, the

relay 25 closes at 25e the hold-on circuit for its second winding, from ground to the armature Hb, the contact of the bank lla, and the corresponding wiper arm of the rotary switch. Ground which was put on by the contact 21a (operative) is transferred by the operative conv tact 25d tothe feed relay 29, the armature 29a of which, when it changes position, reverses the direction of the current flowing through the electro-magnet 30 for controlling the rotary switch 3l. This switch is of the same type as before, i. e. with continuous drive and control by means of an escapement. Consequently the reversal of the direction of the current in the electromagnet 30 causes all the wiper arms to be fed one step forwards. At the same time, the opening of the contact 25a has the effect of breaking the holdon circuit of the relays 24 which become inoperative.

As soon as the rotation is completed, the ground put on by the armature Ilb is no longer on the contact 25e since the rotary switch has moved one step forwards. Consequently, the relay 25 is no longer excited and becomes inoperative. The return of the relay 24 to the inoperative position also causes the relay 21 to return to the inoperative position, so that, as

ground is no longer connected to 27a, the relay 25 remains inoperative.

During all these operations, the circuit of the relays l1 and I8 has remained open (in order to prevent any disturbance caused by the premature reception of a starting frequency) owing to the hold-on relay 32 which was energized by the ground put on by the armature I'lb and which held the contact 32a open. This arrangement is advantageous for short distance communication in which the time of propagation may be less than the time for recording. At the end of a rotation, the ground from Ilb is eliminated, so that the relay 32 immediately becomes inoperative. Consequently, as soon as a new starting frequency arrives, the relays il and i8 move their armatures again.

The rotary switch is provided with a special bank of contacts for the control of relays such as Ln, which energlse at the level n+1 the reading electromagnet for the translating device Tn. The rotary switch therefore has a total of seven rings of contacts.

Itl will be noted that each of the relays 24 is actuated independently of its neighbours and is locked as soon as the corresponding modulation frequency is recorded by the relays 20. It follows therefore that the various groups of relays I9-20 can operate at different instants without reception being disturbed thereby. This protects the transmission from the eifects of selective fading and furthermore enables, as mentioned hereinbefore the various modulation frequencies to be transmitted successively and not simultaneously, for example by means of a 12 rotating distributor. Similarly, it will be readily understood that it is not necessary for the control frequency jto be transmitted continuously; it is only necessary for it to be sent out as soon as the five relays 24 have operated.

The sending back of the control frequency is effected through theline 23 having an adjustable time constant, which enables the speed of the traffic to be varied.` Also a three-way switch 33 stops the sending back of the control frequency thereby stopping the traffic, or allows it to be operated step by step for testing.

It will be seen that each element of the combination transmitted is received at a suitable instant and independently of the others. The system is therefore protected from total or selective fading and also from static. On the other hand, a fresh combination can only be registered if there has been a reversal of the starting frequency. Protection is therefore afforded from echo and other phenomena of the same nature e. g. lengthening of the signals.

The timewhich elapses between two recordings is substantially equal to D 0 -m-ll5 milliseconds where D is the distance in kilometres between two stations in radio communication. For a distance of 1,500 kilometres, for example, 0=25 milliseconds, which provides a traic of 400 words per minute on l2 channels, i. e. 33.3 words per minute per channel, which corresponds to the present average capacity of translating de- Vices and transmitters. In this case, the rotary switches would have to rotate at 40 steps per second, which isl perfectly feasible for apparatuses of the type indicated (pawl escapement type).

The telegraph speed on each frequency would be 40 bauds.

For a much longer distance, for example 15,000 km. 0=115 milliseconds. In this case, the traffic would fall to 86 Words per minute which could be distributed over only 3 channels. But the telegraph speed would only be 8.6 bauds; this would be a case in which it would be advantageous to use at the transmitting end a distributor as referred to hereinbefore, in order to transmit the frequencies successively and relieve the transmitter. Y

With frequencies of (2m-H) 60 of harmonic telegraphy, the width of the band utilised is 2.1 kilocycles. It is therefore possible to place two similar systems side by side, and, on theY assumption that the conditions of propagation are substantially the same on both, it is possible, if the same speed of traine is accepted for each, to use only one control frequency and one starting frequency. The number of frequencies necessary for two systems would then be reduced to 24 and the band width would be 3.18 kilocycles. Finally, with a transmitter having a single sideband, it would be possible to have four similar systems with 22 separate frequencies, by sending the starting frequency and two groups on one side of the carrier and the control frequency and the two other groups on the other. The width of each side-band would only be 2.95 kilocycles,

` and it would be possible to send 1,600 words per minute over a distance of 1,500 kilometres.

In any case, the number of frequencies modulating the transmitter at each instantie constant, thus enabling the depth of modulation to be adjusted to a maximum enabling operation mitter for transmitting an identification signal that identifies the signal character that is being transmitted at the same time as said signal` char- 'acter is transmitted, a receiving station for receiving said signal character and said identification signal, a secondtransmitter connected to said receiving station and controlled thereby for transmitting a control signal to said first transmitter indicating that the transmission of said signal character and the accompanying identification signal have been received at said receiving station, a receiver at said 'first'transmitter for receiving said control signal, said last mentioned receiver having means for causing said rst transmitter to transmit the next `signa-l character and another and difierent identication signal when said control signal is received by said receiver from said second transmitter and means at said first transmitter to prevent the transmission of said next signal character and said other and different identification' signal until said receiving station receives said first mentioned signal character and identification signal and said second transmitter transmits said control signal to said receiver at said iirst transmitter.

2. A telegraphic transmission system in which signal elements are transmitted decomposed into a nite number of elementary characters, comprising a first transmitter for the transmission of the signal characters, means in said transmitter for transmitting an identification signal .identification` signal,

. 14 said signal character is transmitted, a receiving station for receiving said signal character and said identification signal, lter means in said receiving station for selecting the identification signal frequency, a monitor transmitter connected to said receiving station and controlled thereby for transmitting a control signal to said iirst transmitter indicating that the transmis- Sion of said signal character and the accompanying identification signal have been received at said receiving station, a receiver at said first transmitter for receiving said control signal, said last mentioned receiver having means for `causing said lirst transmitter to transmit the next signal character and another identification Signal on the other of said pair of frequencies when "said last mentioned control signa-l is received by-said receiver from said monitor transmitter and means at said first transmitter to prevent the transmission of said signal character and said other identification signal until said receiving station receives said first mentioned signal character1 and identification signal and Asaid monitor transmitter transmits said control signal to said receiver at said rst transmitter.

4. In a' duplex telegraph system, the combination of a transmitter including means for transmitting selectively a frequency from each pair of a double series of frequencies in accordance with a predetermined code to form a coded signal, means in said transmitter for sending fout an identification signal accompanying each coded signal, means for alternately selecting the frequency of said identification signal from a group of two frequencies, a receiver, a monitor transmitter controlled by said receiver to return to said transmitter a control signal when said receiver has received said coded signal and said means in said monitor transmitter for alternately selecting said control signal from a group of two frequencies, two operative circuits respectively at said transmitter and at said receiver, means for giving said operf ative circuits an alternate position and polarity for each successive coded signal through the y action respectively of said control and said identransmitting a control signal to said first trans- .mitter indicating that the transmission of said signal character and the accompanying identification signal have been received at said receiving station, a receiver at said rst transmit-A ter for receiving said control signal, said last.

mentioned receiver having means for causing said rst transmitter to transmit the next signal character and another identilicatonsignal when said control signal is received by said receiver from said monitor transmitter and means at.`

said rst transmitter to prevent the transmission of said next signal character and said other identification signal until said receiving station receives said first mentioned signal character and identification signal and said monitor transmit- 9 ter transmits said control signal to said receiver tification signals, said last means including a relay in said receiver and a relay in said transmitter respectively for controlling the polarity ofV the said operative circuits, a switch in series vter for transmitting an identication signal that identifies the signal character that is being transmitted substantially at the same time as "said signal character is transmitted, means for alternately selecting said identification signal from a pair of two frequencies, a receiver at said first transmittenfor receiving control signals for said transmitter, a receiving station for receiving said signal character and said identification signal, a monitor transmitter connected to said receiving station and controlled thereby for transmitting a control signal consisting oi one of said pair of frequencies to said receiver at -said first transmitter indicating that the transmission of said signal character and the accompanying identiiication signal have been received at said receiving station, and means at said first transmitter for causing said rst transmitter to transmit the next signal character and another identification signal when said control signal is received thereby from said monitor transmitter.

6. A telegraphic transmission system in which signal elements are transmitted decomposed into a finite number of elementary characters, comprising a first transmitter for transmission of signal characters, said transmitter having a plurality of modulation frequency generators, a plurality of switches for selecting the modulation frequencies and a translating device for controlling said switches in accordance with the message to be transmitted, identifying modulating means for modulating said transmitter in accordance with identifying signals for identifying the signal character being transmitted, a control receiver for said transmitter, said control receiver having iilters for selecting control signals, relays connected to each of said lters, a polarized circuit connected so that said relays control the polarity thereof, and means for controlling said identifying modulating means in accordance with the polarity of said polarized circuit.

7. A telegraphic transmission system in which signal elements are transmitted decomposed into a nite number of elementary characters, comprising a first transmitter for the transmission of the `signal characters, means in said transmitter for transmitting an identification signal comprising one of a pair of frequencies that identies the signal character that is being transmitted, a receiving station for receiving said signal character and said identification signal, said receiving station having a pair of filters for selecting said pair of frequencies, a monitor transmitter connected to said receiving station, means for alternately modulating said monitor transmitter in accordance with said pair of frequencies so that said frequencies are received as identification signals by said receiving station and transmitted by said monitor transmitter alternately as control signals to said first transmitter indicating that the transmissionof said signal character and the .accompanying identification signal have been received at said receiving station, a receiver at said first transmitter for receiving said control signal, said last mentioned receiver having means for causing said first transmitter to transmit the next signal character and another identification signal when said control signal is received by said receiver from said monitor transmitter and means at said first transmitter to prevent the transmission of said next signal character and said other identification signal until said receiving station receives said iirst mentioned signal character' and identiication signal and said monitor transmitter transmits said control signal to said receiver at said rst transmitter.

8. A telegraphic transmission system in which signal elements are transmitted decomposed into a finite number of elementary characters, comprising a r-st transmitter` for,A transmission Vof signal characters, said transmitter having a plurality of modulation frequency generators, a plurality of switches for selecting the modulation frequencies and a translating device for controlling said switches in accordance with the messagel to be transmitted, identifying modulating means for modulating said transmitter in accordance with identifying signals for identifying the signal character being transmitted, a control receiver for said transmitter, said control receiver having filters for selecting control signals, relays connected to each of said filters, a polarized circuit connected so that said relays control the polarity thereof, means for controlling said identifying modulating means in accordance with the polarity of said polarized circuit, a receiving station for receiving said signal characters and said identifying signals, said receiving station having filters for said identifying Vsignala'r'elays connected to said nlters, a polarized circuit connected to said last mentioned relays so that said last mentioned relays control the polarity thereof, and a monitor transmitter for transmitting control signals to said iirst transmitter in accordance with the polarity of Vsaid last mentioned polarized circuit.

9. A telegraphic transmission system in which signal elements are transmitted decomposed into a finite number of elementary characters, cornprising a iirst transmitter for transmission of signal characters and identifying signals, said transmitter having a plurality of modulation frequency generators, a plurality of switches for selecting the modulation frequencies and a translating device for controlling said switches in accordance with the message to be transmitted. identifying signal modulating means for modulating said transmitter in accordance with a pair of frequencies for identifying the signal character being transmitted, a control receiver for said transmitter, said control receiver having filters for selecting control signals, relays connected to each of said filters, a polarized circuit connected so that said relays control the polarity thereof, means for controlling said identifying signal modulating means in accordance with the polarity of said polarized circuit to select one or the other of said pair of frequencies depending upon the polarity of said polarized circuit, a receiving station for receiving said signal characters and said identifying signals, means for determining which of said pair of frequencies was PHILIPPE LE oALvEz.

REFERENCES CITED The following references are of record in the file of this patent:

' UNITED STATES PATENTS Number Name Date 2,235,755 Bakker Mar. 18, 1941 

